The regs are that from the factory the windshield has to allow 76% light transmission and all of the other windows 70%. The only exception to that is the rear door or quarter windows and backlight which can have a privacy glass option. Privacy glass is in the region of 15-20% LT.
Once in the hands of the customer I don't know what the legal requirements are for after market films, I suspect that as long as the windshield is clear then you won't be attracting the wrong sort of attention 8)